1. Field of the Invention
The present invention relates to medical devices and techniques, and more particularly to catheter systems and medical interventions for treating elongate occluded regions in vascular coronary artery grafts. The inventive catheter system is adapted for capturing and extracting embolic fragments that typically develop in any endovascular intervention. The catheter system utilizes sequences of micro-electrical discharges in catheter channels to (i) remove occlusive materials from the vessel walls around a treatment site; (ii) generate fluid extraction forces; and (iii) to emulsify or ablate large fragments of dislodged occlusive materials, thus allowing the emboli extraction system to be fabricated in a very small diameter catheter.
2. Description of Related Art
Vascular grafts in coronary artery bypass procedures (CABG) often become occluded over time by plaque, thrombus, or other deposits that can significantly reduce blood flow through the graft. In such bypass grafts, the occlusions are frequently diffuse and elongate making medical interventions problematic. It has been found that conventional treatments of such saphenous vein bypass grafts (e.g., balloon angioplasty, atherectomy, etc.) can cause significant risk of embolisms by dislodging occlusive material that can then migrate downstream. If an embolism occurs at a critical location in the patient""s circulatory system, a permanent injury or even death may occur.
The risk of embolism also is prevalent in medical interventions to treat occlusions in native vessels. For example, it is known that stent deployment often leads to the dislodgement of embolic fragments. In some occlusions in native vessels, such as the carotid arteries, the risks of emboli reaching the brain are so significant that catheter-based treatments of such occlusions are rarely practiced.
Various endovascular catheter systems have been developed for treating occluded vascular grafts and for capturing embolic material during the intervention. As an example, it is believed that the leading candidate for commercialization is an assembly of concentric catheter sleeves that allows for irrigation and aspiration of fluids from a vessel that is temporarily blocked upstream and downstream by inflatable balloons, using an assembly of catheter sleeves as depicted in FIG. 1A. The irrigation and aspiration systems provide a looped flow of fluid (e.g., saline) through inflow and outflow pathways between the various catheter sleeves wherein the pathways communicate with external positive and negative pressure sources. The arrangement of concentric catheters of FIG. 1A was disclosed by Zadno-Azizi et al. in U.S. Pat. No. 6,022,336. In using the type of catheter assembly just described, the physician is supposed to use the intermediate catheter to carry an additional functional component for performing a medical intervention, such as an angioplasty or any other form of treatment. Such treatments may dislodge emboli between the upstream and downstream balloons. The contemporaneous in-and-out irrigation and aspiration of fluids is then intended to flush any embolic particles from the treatment region between the balloons. In order understand the shortcomings of this type of catheter assembly that relies on external irrigation ration systems, it first is necessary to describe the operating parameters of a typical interventionxe2x80x94in terms of (i) the dimensions of the operating space within the vessel and (ii) the dimensions of potential emboli that must be captured and removed
The principal difficulty in designing an interventional catheter system for controlling emboli in vascular grafts relates to the small size of a typical bypass graft. A saphenous vein graft in a CABG procedre has a lumen diameter ranging between about 3 mm. and 4 mm., although some grafts can range to about 5 mm. to 6 mm. Thus, the outside diameter of a catheter system must be small enough to navigate 3 mm. or 4 mm. lumens, and preferably much smaller to treat vessels with smaller lumens, and to pass through the partially occluded lumen of a graft.
The other important consideration in such an intervention treatment relates to the crosssectional dimensions of potential embolic fragments. It is postulated that the most dangerous emboli have cross-sections ranging from greater that a few hundred micrometers xcexcm), for example, from about 200 xcexcm or 300 xcexcm to about 600 xcexcm Certainly, smaller emboli are common and also are targeted for capture and removalxe2x80x94but particles in the range of 50 xcexcm or less may not prove as dangerous as larger emboli since they may pass through the blood stream. Therefore, the catheter system of the type shown in FIG. 1A ideally would have an emboli extraction pathway with a cross-section capable of extracting 500 xcexcm to 600 xcexcm particles without clogging.
As one can easily understand, it is problematic to construct a catheter assembly that has an outer diameter of significantly less than about 3 mm. and still provide an inner lumen diameter of 600 xcexcm or more for extracting embolixe2x80x94particularly when the catheter will require as many as four other inflow/outflow channels or lumens. A second channel will be required for irrigation; third and fourth channels will be required for inflating the proximal and distal balloons; and in most cases, a fifth channel will be required to accommodate a guidewire. Another sixth channel within the working end will be required if the intervention is time-consuming so that blood perfusion around the balloon assembly is needed. Even if the desired functionality associated with the above described five or more lumens could be packaged in a 3.0 mm. cross-section catheter, the overall system still would be much larger than optimal. As can be seen in FIG. 1A, the external dimension of the outer catheter is too large to for easy navigation through a typical blood vessel targeted for treatment. The system of FIG. 1A also has several other serious drawbacks, as will be described next.
In order to better understand the functionality of the prior art catheter assembly of FIG. 1A, it is necessary to explain the parameters for providing xe2x80x9coptimized paths for irrigation and aspirationxe2x80x9d as proposed in U.S. Pat. No. 6,022,336. Further, it is necessary to analyze real-world dimensions of a typical treatment space. For this reason, TABLE A is provided below, which along with FIGS. 1A and 1B, describe the practical dimensions of an exemplary catheter assembly of the type proposed in U.S. Pat. No. 6,022,336.
The figures in TABLE A are listed in or xcexcm (i.e., micrometers with an approximation in inches) to allow reference to emboli dimensions which are typically given in micrometers. For the catheter assembly of FIGS. 1A-1B to function optimally in capturing and removing emboli, there are essentially three dimensional factors that must be considered: (i) the radial dimension of the free space between the catheter""s exterior and the vessel wall to allow fluid flows therein to remove, capture and entrain emboli; (ii) the cross-sectional dimension of the fluid irrigation pathway within the catheter assembly to allow sufficient fluid inflows; and (iii) most importantly, the cross-sectional dimension of the extraction pathway within the catheter assembly to allow embolic fragment to flow therethrough to the remote handle of the catheter.
As discussed above, consider that the typical vessel lumen targeted for treatment is either 3 mm. or 4 mm. (i.e., 3000 xcexcm to 4000 xcexcm) as indicated in TABLE A The dimensions of the exemplary catheter assembly are best aggregated from the inner catheter outwardly. The inner catheter sleeve indicated at 4 in FIGS. 1A-1B has a lumen diameter of 500 xcexcm to place over a typical 0.014xe2x80x3 guidewire. As can be seen in FIG. 1A, this catheter sleeve 4 requires a balloon inflation lumen in a thickened wall portion that results in an outer catheter sleeve diameter of about 960 xcexcm. FIG. 1A shows the irrigation pathway comprising by the free space between the inner catheter sleeve 4 and the intermediate catheter sleeve 5, which is best measured by a difference (xcex94) in the radial dimension (or radius) between the respective sleeve diameters. In this exemplary embodiment, the radial xcex94 is 120 xcexcm which can provide sufficient cross-section area for projected fluid inflow rates. The intermediate catheter sleeve 5 thus has an inner diameter of about of 1200 xcexcm. The very thin wall around the lumen of the catheter (e.g., about 200 xcexcm) results in an outer diameter of 1600 xcexcm for the intermediate catheter sleeve 5.
In the assembly of FIG. 1A, the aspiration or extracion passageway must be located between the outer catheter sleeve 6 and the intermediate catheter sleeve 5. Assume that the outer diameter (OD) of the outer catheter sleeve 6 is at a maximum possible dimension, for example 2800 xcexcm which is close to the diameter of the lumen targeted for treatment. Still referring to FIG. 1A, the outer catheter sleeve 6 further requires a balloon inflation lumen in a wall portion thereof resulting in a maximum inner sleeve diameter of about 2300 xcexcm. Now, it can be seen that the radial xcex94 of the aspiration passageway in this embodiment is only about 350 xcexcm
A number of important practical observations can be made from analyzing the data in TABLE A and FIGS. 1A-1B. Most important, many of the most dangerous embolic particles with large dimensions (e.g., 400 xcexcm to 600 xcexcm) could not be carried through the aspiration pathway which has a radial A of only 350 xcexcm. The outermost diameter of the catheter assembly cannot realistically be increased since it already is 2800 xcexcmxe2x80x94close to the diameter of the vessel lumen targeted for treatment. TABLE A further shows that the free space on either side of the catheter assembly in the vessel lumen has a radial xcex94 of 350 xcexcm to 1200 xcexcm, for 3 mm. and 4 mm. vessels, respectively. This open dimension of the working space on one side of the catheter may be too small to carry large embolic fragments to the open end of the aspiration passageway. In any such cases that the emboli is not aspirated, the large emboli would remain in the vessel lumen after the downstream balloon is collapsed, and thereafter would be free to migrate downstream and cause an embolism.
Besides the fact that a 2.8 mm. catheter assembly of FIG. 1A cannot optimize dimensions for (i) the working space, (ii) the irrigation pathway, and (iii) the emboli aspiration pathway, there are several other technical reasons that limit the suitability of such a catheter assembly for containing and removing emboli. First the entire purpose of the catheter assembly of FIG. 1A is to provide the intermediate catheter 5 with additional medical intervention functionality, such as any angioplasty, stent delivery or atherectomy means. For example, if a typical balloon angioplasty system were added to the intermediate catheter 5, an additional high-pressure balloon inflation lumen would be required which would result in much thicker walls in the intermediate catheter, and correspondingly greater dimensions in the outer catheter (or lesser dimensions of the aspiration pathway). Similarly, if any other functional components were added to the intermediate catheter, its cross-section would be increased. Thus, in a real world fabrication of a working catheter assembly as shown in FIG. 1A, the free working space, the irrigation passageway and/or the aspiration passageway would be much further restricted by the increased cross-section of a true working intermediate catheter.
Second, the catheter""s looped fluid flow from the irrigation-aspiration sources can function properly only with a balance of inflow and outflow pressure levels from the external sources used to circulate fluid through the working space. Such positive and negative pressures sources must be finely balanced so as to not cause overpressures in the working space. Any overpressure of about two to three times the normal intravascular pressure could rupture the vessel wall within the working space isolated between the balloons. It can be easily understood that the looped fluid flowxe2x80x94first distally (inwardly) in the irrigation pathway and then outwardly (proximally) in the aspiration pathwayxe2x80x94creates inertial forces. If the aspiration channel were suddenly blocked, significant overpressures would instantly build up in the working space. It firer can be seen that blockages of the aspiration channel of the catheter assembly of FIG. 1A are highly likely. As mentioned above, large emboli may not even pass through the aspiration channel; other smaller emboli may still clog the aspiration pathway since the shape of the concentric area around the intermediate catheter 5 will be in flux as the intermediate catheter sleeve flexes with the constraining bore of the outer catheter 6, which is indicated by the arrows in FIG. 1B. As can be understood from FIGS. 1A-1B, even slight changes in the crosssections of the irrigation and aspiration pathways will make it difficult to modulate pressures to maintain the looped circulatory flow at a particular pressure. More likely, there will be pressure spikes in the working space that could risk rupturing the vessel and risk the patient""s health.
A third serious drawback of the catheter assembly of FIG. 1A relates to its lack of flexibility. Many, if not all, targeted sites will be found in vessels that are curved or even tortuous. Since the catheter assembly may substantially occupy the lumen of the vessel, and must flex, one side of the outer catheter sleeve 6 inevitably will be pressed against the vessel wall. Thus, the looped circulatory flows of fluids with the assembly of FIG. 1A will not typically cleanse fragments from the vessel wall portion that is pressed against the catheter. Due to the limited treatment time in which fluid flows about the working space (i.e., as little as 2 to 4 minutes since downstream blood flow is blocked by the balloon) there probably will be inadequate time to adjust the catheter in some way to insure that fluid flows reach all wall portions of the vessel. As mentioned previously, it would be possible to increase treatment times by adding a perfusion lumen. However, it would very difficult to carry blood through three different catheter sleeves and then around the paired occlusion balloons. Any such perfusion functionality would likely require a much larger assembly cross-section, which is not a realistic option.
The author believes that principles of irrigation and aspiration of fluids to remove emboli from an endovascular workspace are sensible. However, the prior art catheter assembly of FIG. 1A is best suited for larger vessels, for example, vessels having a lumen of about 5 to 6 mm. or larger.
What is needed is a catheter system for containing and removing emboli that has the following characteristics:
(i) a system that can be scaled down in cross-section to function in 2 mm. to 3 mm. lumens, or even smaller;
(ii) a system that can effectively remove large embolixe2x80x94up to 600 micrometers in diameter or larger;
(iii) a system that eliminates any possibility of overpressure in an endoluminal working space;
(iv) an endovascular system that cooperates with related interventions (e.g., angioplasty or stent placement)
(v) an endovascular system that can remove any type of embolic material, from calcified fragments to thrombus; and
vi) an endovascular system that allows for optional blood perfusion to allow an increase in treatment time.
The catheter system of the present invention provides novel occlusive material extraction (removal) techniques that utilize sequences of very small electrical discharges between paired electrodes in a fluid-jet arrangement (i) to generate selected fluid flow velocities in a fluid extraction pathway based on Bernoulli""s Law of Pressure Differential, (ii) to create selected levels of turbulent fluid flow within a treatment site to remove occlusive material from the vessel walls and suction fluids and entrained embolic particles toward the extraction pathway, and (iii) to emulsify or ablate any embolic particles having a cross-sectional dimension larger than a couple of hundred micrometers.
The novel energy delivery methods associated with the sequenced electrical discharges allow for the design of a catheter working end that has a significantly reduced cross-sectional dimension, when compared to prior art system (cf. FIG. 1A). The catheter system according to invention has about 50% (or less) of the cross-section of the prior art systems described above. Still, the present invention can provide an embolic extraction pathway that has a significantly larger functional cross-section than the prior art system shown in FIG. 1Axe2x80x94as much as 200% to 300% larger.
More in particular, the invention provides an elongate microcatheter sleeve with a distal working end that can be passed through a lumen in tubular anatomic structure to reach a targeted endoluminal site. The catheter sleeve defines an interior passageway surrounded by a catheter wall. The interior passageway may be termed an extraction channel since it is adapted to extract fluid and embolic particles from the endoluminal site. The wall of the catheter sleeve carries small diameter channels (or microchannels) with first and second electrodes for accelerating, or jetting, fluid flows in the proximal direction through an open terminus into the extraction passageway. The electrode carrying channels have a preferred cross-sectional dimension ranging from less than 1 xcexcm to about 1000 xcexcm and communicate with a remote fluid media inflow source. The catheter system provides an electrical source coupled to the first and second electrodes together with a computer controller for controlling parameters of electrical discharges between the electrodes, such as the power delivered by an electrical discharge, the profile of energy delivery within a discharge, the length of a discharge and the repetition rate of discharges.
For treating an endoluminal site, for example a partially occluded vascular graft, the physician advances the working end of the catheter to the targeted site. Next, the physician expands the upstream and downstream balloons carried by the working end to occlude the lumen. The endovascular space between the occlusion balloons thus isolates a working space and prevents any embolic particles from migrating downstream. Thereafter, the physician actuates the controller to cause electrical discharges of a sped power and repetition rate within fluid flows in the microchannels thereby causing high velocity flows into the extraction passageway. This aspect of the method of the invention, based on Bernoulli""s Law of Pressure Differential, causes a selected level of turbulent fluid flow within the working space as fluids are suctioned toward the extraction passageway. The turbulent fluid flows are used to remove occlusive materials that adhere to the vessel wall. Another aspect of the method of the invention relates to the delivery of mechanical energy in the form acoustic waves to the embolic particles within the fluid extraction pathway that are proximate to the fluid jets. This form of energy delivery, as well as thermal energy delivery, can emulsify, fragment or ablate any embolic particles having a relatively large cross-sectional dimension. The system also can optionally deliver a pharmacological agent (e.g., t-PA) to the working space to further dissolve any particles, which is herein defined as a method of delivering chemical energy to occlusive material within, or remove from, the targeted site.
The fact that the microcatheter comprises a single sleeve member that defines a central extraction passageway distinguishes the invention from the prior art catheter system described above. The microcatheter of the invention only requires two small crosssection fluid carrying lumens in the catheter wall, thus allowing the catheter assembly to have a very small outside diameter. Referring to TABLE B below, one embodiment of catheter sleeve can have an outside diameter of about 1450 xcexcm, which is approximately 50% of the diameter of the prior art outer catheter described in TABLE A above. At the same time, the cross section of the extraction passageway of the present invention can be about 1000 xcexcm, or close to 3 times the diameter of the prior art system (cf. TABLE A). Further, the free working space in the targeted site is larger when using the system of the invention.
The small diameter of the working end of the catheter allows it to be used in smaller vessels, as well as to extract and process larger embolic fragments without clogging the extraction passageway. Further, the catheter sleeve of the present invention can be very flexible, in contrast to the prior art systems, to allow the sleeve to access treatment sites in tortuous vessels. Numerous other advantages result from the reduction in diameter of the catheter working end enabled by the present invention.
The catheter system of the present invention advantageously uses electrical discharges to cause high velocity fluid flows to create extraction forces at the catheter working end in accordance with Bemoulli""s Law of Pressure Differential to extract fluids and embolic particles.
The catheter system provide fluid flows and fluid turbulence to remove occlusive materials from the vessel walls around a targeted site.
The catheter system advantageously creates high-pressure fluid flows to delivery energy to fluids and entrained particles to emulsify, fragment and ablate embolic particles.
The catheter system advantageously creates cavitation bubbles in fluids in an extraction channel to cause acoustic energy to emulsify occlusive material.
The catheter system provides a fluid media having a selected resistivity for controlling the velocity of fluid flows by altering the effect of an electrical discharge.
The catheter system of the invention provides extraction forces within a working space isolated by first and second occlusion balloons to prevent embolic particles from migrating downstream.
The catheter system advantageously creates a selected high pressure differential by expanding the volume of a fluid media in a confining channel in a phase state change.
The catheter system creates a high pressure jetting effect to create selected fluid velocities in a preferred range of about 1 m/s to about 10 m/s.
The catheter system advantageously utilizes an electrical energy source for delivering electrical discharges to the working end that is reliable and inexpensive.